The six-year old U.S. outbreak of West Nile virus is a significant threat to public health and shows no signs of abating. Last year, there were more than 2,500 serious cases and 100 deaths. Still early in this year's West Nile virus season (there is a time lag during which animals are infected, mosquitoes convey the virus to humans, and the virus incubates until symptoms occur), the mosquito-borne virus has been found in animal hosts (primarily birds) in 44 states, and has caused almost a thousand serious infections and a score of deaths in humans in 36 states. As of...